Inks are generally a liquid with a dye dissolved in them. Inks are also known which comprise a liquid and particles dispersed in the ink. The actual composition of machine inks as distinct from writing inks is determined by the type of application method for such inks.
There are two prime types of ink jet application techniques in use today. The first of these is known as continuous ink jet and the second is impulse ink jet.
In continuous ink jet systems, droplets of ink are produced as a result of hydrostatic pressure on a liquid ink forcing said ink through a nozzle. The nozzle can be stimulated, such as by piezo-electric vibration to cause the droplets to be ejected on a regular basis. At the point of formation, the droplets may be conductively and/or electrochemically charged and subsequently pass through electrodes which provide a constant electrostatic field causing these droplets to be deflected. Therefore the required droplets are controlled to a substrate and unwanted ones are removed. Due to the charging requirements it is necessary that the ink be conductive. Such an ink may be water or solvent based and the colorant may be a pigment or soluble dye.
In the impulse ink jet system, droplets are produced only as required by the generation of a pressure disturbance within the ink adjacent to a jet nozzle. The application of pressure may be by stimulation caused by a piezo-electric crystal or by thermal stimulation where a micro bubble is created by an electrically resistive heater. Typical inks are water based and use a dye for the colorant though the use of solvents and pigments are not precluded for this process. As there is no need for droplet charging there is no specific requirement concerning conductivity of the ink.
A further embodiment of the impulse system is hot melt ink jet technology. This process is similar to the piezo-electric drop on demand except that the ink is solid at room temperature but is maintained at an elevated temperature in the print head such that it is a liquid under operating conditions. Again therefore, there is no need for the ink's conductivity to be suited to some equipment performance standard.
A final system that utilises small electromechanical valves to produce droplets is known as the valve jet process. This is in essence a micro spray gun system and produces much larger droplets than the true ink jet technologies. As the droplets are produced mechanically there is no requirement concerning the conductivity of the ink.
The preceding technologies are limited in terms of achievable resolution due to droplet formation being a function of the jet nozzle diameter. The smallest nozzles used have diameter in the range of 7.5 to 10 micrometers which produce droplets of the order of 14 to 20 micrometers and this translates to a dot size of 30 to 40 micrometers on selected papers. Further, the inks used in these small nozzle jet printers are based on water soluble dyes, as pigment based inks cause problems with nozzle blocking. Therefore the advantages of pigment formulations, namely better waterfastness and lightfastness as well as a broader range of available colours, are precluded.
Electrostatic ink jet is a further technology which is characterised by droplets being drawn from an orifice under the influence of an electrostatic field. This field acting between a valving electrode and the orifice, attracts free charges within the ink to its surface such that a droplet is produced when the electrostatic pull exceeds the surface tension of the ink. As this technique relies on attraction of free charges, it therefore requires that the ink be conductive.
A new ink jet printing technology has been described in WO-A-9311866 in the name of Research Laboratories Of Australia published on the 24 th of June 1993. This process provides a means of producing variable sized droplets that contain a high concentration of particulate material.
Specific advantages conveyed by this process include the ability to form droplets as small as a few micrometers while still using pigments as the colorant material. This is because the size of the droplets are controlled primarily by the voltage on an ejection point plus the ability of the particles to be charged and so, are not limited by the size of an ink jet nozzle. Also the colorant material is significantly concentrated in the ejected droplets. Therefore high resolution and high density images based on light and water resistant pigments can be produced.